Replacement gate having work function at valence band edge

ABSTRACT

Replacement gate stacks are provided, which increase the work function of the gate electrode of a p-type field effect transistor (PFET). In one embodiment, the work function metal stack includes a titanium-oxide-nitride layer located between a lower titanium nitride layer and an upper titanium nitride layer. The stack of the lower titanium nitride layer, the titanium-oxide-nitride layer, and the upper titanium nitride layer produces the unexpected result of increasing the work function of the work function metal stack significantly. In another embodiment, the work function metal stack includes an aluminum layer deposited at a temperature not greater than 420° C. The aluminum layer deposited at a temperature not greater than 420° C. produces the unexpected result of increasing the work function of the work function metal stack significantly.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.12/948,031, filed Nov. 17, 2010 the entire content and disclosure ofwhich is incorporated herein by reference.

BACKGROUND

The present disclosure generally relates to semiconductor devices, andparticularly to semiconductor structures having dual work function metalgates and a high-k gate dielectric, and methods of manufacturing thesame.

High gate leakage current of silicon oxide and nitrided silicon dioxideas well as depletion effect of polysilicon gate electrodes limits theperformance of conventional semiconductor oxide based gate electrodes.High performance devices for an equivalent oxide thickness (EOT) lessthan 1 nm require high dielectric constant (high-k) gate dielectrics andmetal gate electrodes to limit the gate leakage current and provide highon-currents. Materials for high-k gate dielectrics include ZrO₂, HfO₂,other dielectric metal oxides, alloys thereof, and their silicatealloys.

In general, dual metal gate complementary metal oxide semiconductor(CMOS) integration schemes employ two gate materials, one having a workfunction near the valence band edge of the semiconductor material in thechannel and the other having a work function near the conduction bandedge of the same semiconductor material. In CMOS devices having asilicon channel, a conductive material having a work function of 4.0 eVis necessary for n-type metal oxide semiconductor field effecttransistors (NMOSFETs, or “NFETs”) and another conductive materialhaving a work function of 5.1 eV is necessary for p-type metal oxidesemiconductor field effect transistors (PMOSFETs, or “PFETs”). Inconventional CMOS devices employing polysilicon gate materials, aheavily p-doped polysilicon gate and a heavily n-doped polysilicon gateare employed to address the needs. In CMOS devices employing high-k gatedielectric materials, two types of gate stacks comprising suitablematerials satisfying the work function requirements are needed for thePFETs and for the NFETS, in which the gate stack for the PFETs providesa flat band voltage closer to the valence band edge of the material ofthe channel of the PFETs, and the gate stack for the NFETs provides aflat band voltage closer to the conduction band edge of the material ofthe channel of the NFETs. In other words, threshold voltages need to beoptimized differently between the PFETs and the NFETs.

A challenge in semiconductor technology has been to provide two types ofgate electrodes having a first work function at or near the valence bandedge and a second work function at or near the conduction band edge ofthe underlying semiconductor material such as silicon. This challengehas been particularly difficult because the two types of gate electrodesare also required to be a metallic material having a high electricalconductivity.

SUMMARY

Replacement gate stacks are provided, which increase the work functionof the gate electrode of a p-type field effect transistor (PFET) so thatthe work function approaches the energy level of the valence band edgeof silicon. In one embodiment, the work function metal stack includes atitanium-oxide-nitride layer located between a lower titanium nitridelayer and an upper titanium nitride layer. When formed over underlyingwork function metal layers and below an aluminum layer, the stack of thelower titanium nitride layer, the titanium-oxide-nitride layer, and theupper titanium nitride layer produces an unexpected result of increasingthe work function of the work function metal stack significantly, e.g.,by about 70 mV. In another embodiment, the work function metal stackincludes an aluminum layer deposited at a temperature not greater than420° C., which is lower than conventional deposition temperatures foraluminum layer and significantly reduces reflow of the depositedaluminum material. The aluminum layer deposited at a temperature notgreater than 420° C. produces an unexpected result of increasing thework function of the work function metal stack significantly, e.g., byabout 70 mV. The formation of the titanium-oxide-nitride layer anddeposition of the aluminum layer deposited at a temperature not greaterthan 400° C. can be employed in tandem to increase the work function ofa work function metal stack.

According to an aspect of the present disclosure, a method of forming asemiconductor structure including a field effect transistor is provided.The method includes: forming a disposable gate structure on asemiconductor substrate; forming and planarizing a planarizationdielectric layer, wherein a top surface of the planarization dielectriclayer is coplanar with a top surface of the disposable gate structure;recessing the disposable gate structure to form a gate cavity; andforming a replacement gate stack in the cavity, wherein the replacementgate stack is formed by depositing and patterning a stack including,from bottom to top, a gate dielectric layer, a work function metallayer, at least one barrier metal layer, and an aluminum-includinglayer, wherein the aluminum layer is deposited at a temperature notgreater than 420 degrees Celsius.

According to another aspect of the present disclosure, a method offorming a semiconductor structure including a field effect transistor isprovided. The method includes: forming a disposable gate structure on asemiconductor substrate; forming and planarizing a planarizationdielectric layer, wherein a top surface of the planarization dielectriclayer is coplanar with a top surface of the disposable gate structure;recessing the disposable gate structure to form a gate cavity; andforming a replacement gate stack in the cavity, wherein the replacementgate stack is formed by depositing and patterning a stack including atleast a gate dielectric layer, a lower barrier metal layer including ametal, a metal oxide monolayer including an oxide of the metal andcontacting the lower barrier metal layer, an upper barrier metal layercontacting the metal oxide monolayer.

According to yet another aspect of the present disclosure, asemiconductor structure including a field effect transistor is provided.The field effect transistor contains a gate stack, which includes: agate dielectric located on a semiconductor substrate; a lower barriermetal portion including a metal and located on the gate dielectric; ametal oxide monolayer portion including an oxide of the metal andcontacting the lower barrier metal portion; and an upper barrier metalportion contacting the metal oxide monolayer portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph comparing the work function of various replacementgate stacks as measured on test samples subjected to various oxidationprocesses during formation of the respective replacement gate during aresearch leading to the present disclosure.

FIG. 2 is vertical cross-sectional view of an exemplary semiconductorstructure after formation of disposable gate structures and formation ofa planar dielectric surface on a planarization dielectric layeraccording to an embodiment of the present disclosure.

FIG. 3 is a vertical cross-sectional view of the exemplary semiconductorstructure of FIG. 2 after removal of the disposable gate structures.

FIG. 4 is a vertical cross-sectional view of the exemplary semiconductorstructure of FIG. 3 after formation of a first-type work function metallayer.

FIG. 5 is a vertical cross-sectional view of the exemplary semiconductorstructure of FIG. 4 after application of a photoresist and lithographicpatterning of the first-type work function metal layer.

FIG. 6 is a vertical cross-sectional view of the exemplary semiconductorstructure of FIG. 5 after removal of the photoresist and formation of asecond-type work function metal.

FIG. 7 is a vertical cross-sectional view of the exemplary semiconductorstructure of FIG. 6 after patterning the second-type work function.

FIG. 8 is a vertical cross-sectional view of the exemplary semiconductorstructure of FIG. 7 after deposition of a lower metallic barrier layerand an upper metallic barrier layer.

FIG. 9 is a magnified view of a portion of the upper metallic barrierlayer of FIG. 8.

FIG. 10 is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 8 after deposition of a conductive metallayer.

FIG. 11 is a vertical cross-sectional view of the exemplary structure ofFIG. 10 after planarization.

FIG. 12 is a vertical cross-sectional view of the exemplary structure ofFIG. 11 after formation of a via-level dielectric layer and contact viastructures.

DETAILED DESCRIPTION

As stated above, the present disclosure relates to semiconductorstructures having dual work function metal gates and a high-k gatedielectric, and methods of manufacturing the same, which are nowdescribed in detail with accompanying figures. Like and correspondingelements mentioned herein and illustrated in the drawings are referredto by like reference numerals. The drawings are not necessarily drawn toscale.

Referring to FIG. 1, a graph compares the work function of variousreplacement gate stacks that were tested during the course of researchthat lead to the present disclosure. Test samples each includingdifferent types of replacement gate stacks were subjected to variousoxidation processes. These various samples are labeled as sample Athrough sample O. Sample A included a base film stack, from whichdeviations were introduced in other samples. Specifically, sample Aincluded a gate stack consisting of, from bottom to top, a 2.5 nm thickhafnium oxide gate dielectric, a 2 nm thick titanium nitride workfunction layer, 1.5 nm thick tantalum nitride layer, a 5 nm thicktitanium nitride layer deposited by atomic layer deposition, a 3 nmthick titanium-aluminum alloy layer, a 8 nm thick titanium nitride layerdeposited by atomic layer deposition, and 10 nm titanium layer, and a300 nm thick aluminum layer deposited at 440° C. in two steps ofphysical vapor deposition. Tables 1A and 1B below tabulate the variousdeviations for other samples from the process employed for sample A.

TABLE 1A Processing conditions for lower layers in various replacementgate stacks. Oxidation Tantalum Oxidation Hafnium after TitaniumOxidation nitride after oxide HfO₂ nitride after TiN (TaN) by TaN Sample(HfO₂) deposition (TiN) deposition ALD deposition A 2.5 nm NONE 2 nmNONE 1.5 nm NONE B 2.5 nm 400° C., 10 2 nm NONE 1.5 nm NONE Torr O₂, 1min C 2.5 nm 400° C., 10 2 nm NONE 1.5 nm NONE Torr O₂, 5 min D 2.5 nmNONE 2 nm 400° C., 10 1.5 nm NONE Torr O₂, 1 min E 2.5 nm NONE 2 nm 400°C., 10 1.5 nm NONE Torr O₂, 5 min F 2.5 nm NONE 2 nm NONE 1.5 nm 400°C., 10 Torr O₂, 1 min G 2.5 nm NONE 2 nm NONE 1.5 nm 400° C., 10 TorrO₂, 5 min H 2.5 nm NONE 2 nm NONE 1.5 nm NONE I 2.5 nm NONE 2 nm NONE1.5 nm NONE J 2.5 nm NONE 2 nm NONE 1.5 nm NONE K 2.5 nm NONE 2 nm NONE1.5 nm NONE L 2.5 nm NONE 2 nm NONE 1.5 nm NONE M 2.5 nm NONE 2 nm NONE1.5 nm NONE N 2.5 nm NONE 2 nm NONE 1.5 nm NONE O 2.5 nm NONE 2 nm NONE1.5 nm NONE

TABLE 1B Processing conditions for upper layers in various replacementgate stacks. First Oxidation Titanium- Oxidation Oxidation titaniumAfter TiAl aluminum After TiAl Second after nitride alloy (TiAl) alloyTiN by TiN Sample by ALD deposition alloy deposition ALD deposition A 5nm NONE 3 nm NONE 8 nm NONE B 5 nm NONE 3 nm NONE 8 nm NONE C 5 nm NONE3 nm NONE 8 nm NONE D 5 nm NONE 3 nm NONE 8 nm NONE E 5 nm NONE 3 nmNONE 8 nm NONE F 5 nm NONE 3 nm NONE 8 nm NONE G 5 nm NONE 3 nm NONE 8nm NONE H 5 nm 400° C., 10 3 nm NONE 8 nm NONE Torr O₂, 1 min I 5 nm400° C., 10 3 nm NONE 8 nm NONE Torr O₂, 5 min J 3 nm 400° C., 10 3 nmNONE 8 nm NONE Torr O₂, 5 min K 8 nm 400° C., 10 3 nm NONE 8 nm NONETorr O₂, 5 min L 5 nm NONE 3 nm Exposure to air, 8 nm NONE 30 min, 20°C. M 5 nm NONE 3 nm 400° C., 10 8 nm NONE Torr O₂, 1 min N 5 nm NONE 3nm NONE 8 nm 400° C., 10 Torr O₂, 1 min O 5 nm NONE 3 nm NONE 8 nm 400°C., 10 Torr O₂, 1 min

The 10 nm titanium layer and a 300 m thick aluminum layer deposited at440° C. in two steps of physical vapor deposition were common across alltested samples shown in FIG. 1. FIG. 1 shows that the work function ofsamples B-O is not greater than the work function of the referencesample, i.e., sample A. Thus, the various oxidation processes introducedto samples B-O not only fail to increase the work function of thereplacement gate stacks, but tend to decrease the work function of thevarious tested replacement gate stacks. In general, therefore, oxidizingany portion of a metal layer in a replacement gate stack is likely tohave a detrimental effect of reducing the work function, if the effectis present at all. In other words, reduction of work function isexpected when any metal layer in a replacement gate stack is subjectedto any type of oxidation.

Referring to FIG. 2, an exemplary semiconductor structure according toan embodiment of the present disclosure includes a semiconductorsubstrate 8, on which various components of field effect transistors areformed. The semiconductor substrate 8 can be a bulk substrate includinga bulk semiconductor material throughout, or asemiconductor-on-insulator (SOI) substrate (not shown) containing a topsemiconductor layer, a buried insulator layer located under the topsemiconductor layer, and a bottom semiconductor layer located under theburied insulator layer.

Various portions of the semiconductor material in the semiconductorsubstrate 8 can be doped with electrical dopants of p-type or n-type atdifferent dopant concentration levels. For example, the semiconductorsubstrate 8 may include an underlying semiconductor layer 10, a firstconductivity type well 12B, and a second-conductivity type well 12A. Thefirst conductivity type well 12B is doped with electrical dopants of afirst conductivity type, which can be p-type or n-type. The secondconductivity type well 12A is doped with electrical dopants of a secondconductivity type, which is the opposite type of the first conductivitytype. For example, if the first conductivity type is p-type, the secondconductivity type is n-type, and vice versa.

Shallow trench isolation structures 20 are formed to laterally separateeach of the first conductivity type well 12B and the second conductivitytype well 12A. Typically, each of the first conductivity type well 12Band the second conductivity type well 12A is laterally surrounded by acontiguous portion of the shallow trench isolation structures 20. If thesemiconductor substrate 8 is a semiconductor-on-insulator substrate,bottom surfaces of the first conductivity type well 12B and the secondconductivity type well 12A may contact a buried insulator layer (notshown), which electrically isolates each of the first conductivity typewell 12B and the second conductivity type well 12A from othersemiconductor portions of the semiconductor substrate 8 in conjunctionwith the shallow trench isolation structures 20.

A disposable dielectric layer and a disposable gate material layer aredeposited and lithographically patterned to form disposable gatestructures. For example, the disposable gate stacks may include a firstdisposable gate structure that is a stack of a first disposabledielectric portion 29A and a first disposable gate material portion 27Aand a second disposable gate structure that is a stack of a seconddisposable dielectric portion 29B and a second disposable gate materialportion 27B. The disposable dielectric layer includes a dielectricmaterial such as a semiconductor oxide. The disposable gate materiallayer includes a material that can be subsequently removed selective todielectric material such as a semiconductor material. The firstdisposable gate structure (29A, 27A) is formed over the secondconductivity type well 12A, and the second disposable gate structure(29B, 27B) is formed over the first conductivity type well 12B. Theheight of the first disposable gate structure (29A, 27A) and the seconddisposable gate structure (29B, 27B) can be from 20 nm to 500 nm, andtypically from 40 nm to 250 nm, although lesser and greater heights canalso be employed.

Dopants of the first conductivity type are implanted into portions ofthe second conductivity type well 12A that are not covered by the firstdisposable gate structure (29A, 27A) to form first source and drainextension regions 14A. The first conductivity type well 12B can bemasked by a photoresist (not shown) during the implantation of the firstconductivity type dopants to prevent implantation of the firstconductivity type dopants therein. Similarly, dopants of the secondconductivity type are implanted into portions of the first conductivitytype well 12B that are not covered by the second disposable gatestructure (29B, 27B) to form second source and drain extension regions14B. The second conductivity type well 12A can be masked by aphotoresist (not shown) during the implantation of the secondconductivity type dopants to prevent implantation of the secondconductivity type dopants therein.

Dielectric gate spacers are formed on sidewalls of each of thedisposable gate structures, for example, by deposition of a conformaldielectric material layer and an anisotropic etch. The dielectric gatespacers include a first dielectric gate spacer 52A formed around thefirst disposable gate structure (29A, 27A) and a second dielectric gatespacer 52B formed around the second disposable gate structure (29B,27B).

Dopants of the first conductivity type are implanted into portions ofthe second conductivity type well 12A that are not covered by the firstdisposable gate structure (29A, 27A) and the first dielectric gatespacer 52A to form first source and drain regions 16A. The firstconductivity type well 12B can be masked by a photoresist (not shown)during the implantation of the first conductivity type dopants toprevent implantation of the first conductivity type dopants therein.Similarly, dopants of the second conductivity type are implanted intoportions of the first conductivity type well 12B that are not covered bythe second disposable gate structure (29B, 27B) and the seconddielectric gate spacer 52B to form second source and drain regions 16B.The second conductivity type well 12A can be masked by a photoresist(not shown) during the implantation of the second conductivity typedopants to prevent implantation of the second conductivity type dopantstherein.

In some embodiments, the first source and drain regions 16A and/or thesecond source and drain regions 16B can be formed by replacement of thesemiconductor material in the second conductivity type well 12A and/orthe semiconductor material in the first conductivity type well 12B witha new semiconductor material having a different lattice constant. Inthis case, the new semiconductor material(s) is/are typicallyepitaxially aligned with (a) single crystalline semiconductormaterial(s) of the second conductivity type well 12A and/or thesemiconductor material in the first conductivity type well 12B, andapply/applies a compressive stress or a tensile stress to thesemiconductor material of the second conductivity type well 12A and/orthe semiconductor material in the first conductivity type well 12Bbetween the first source and drain extension regions 14 A and/or betweenthe second source and drain extension regions 14B.

First metal semiconductor alloy portions 46A and second metalsemiconductor alloy portions 46B are formed on exposed semiconductormaterial on the top surface of the semiconductor substrate 8, forexample, by deposition of a metal layer (not shown) and an anneal.Unreacted portions of the metal layer are removed selective to reactedportions of the metal layer. The reacted portions of the metal layerconstitute the metal semiconductor alloy portions (46A, 46B), which caninclude a metal silicide portions if the semiconductor material of thefirst and second source and drain regions (16A, 16B) include silicon.

Optionally, a dielectric liner 54 may be deposited over the metalsemiconductor alloy portions 54, the first and second disposable gatestructures (29A, 27A, 29B, 27B), and the first and second dielectricgate spacers (52A, 52B). A first type stress-generating liner 58 and asecond type stress-generating liner 56 can be formed over the firstdisposable gate structure (29A, 27A) and the second disposable gatestructure (29B, 27B), respectively. The first type stress-generatingliner 58 and/or the second type stress-generating liner 56 can beemployed to apply uniaxial or biaxial lateral stress to a first channelregion, which is the portion of the second conductivity type well 12Abetween the first source and drain extension regions 14A, and/or to asecond channel region, which is the portion of the first conductivitytype well 12B between the second source and drain extension regions 14B,respectively. In one embodiment, one of the first type stress-generatingliner 58 and the second type stress-generating liner 56 applies acompressive stress if underlying source and drain regions (i.e., thefirst source and drain regions 16A or the second source and drainregions 16B) are p-doped regions, and the other of the first typestress-generating liner 58 or the second type stress-generating liner 56applies a tensile stress if underlying source and drain regions (i.e.,the second source and drain regions 16B and the first source and drainregions 16A) are n-doped regions. The first type stress-generating liner58 and the second type stress-generating liner 56 can include adielectric material that generates a compressive stress or a tensilestress to underlying structures, and can be silicon nitride layersdeposited by plasma enhanced chemical vapor deposition under variousplasma conditions.

A planarization dielectric layer 60 is deposited over the first typestress-generating liner 58 and/or the second type stress-generatingliner 56, if present, or over the metal semiconductor alloy portions 54,the first and second disposable gate structures (29A, 27A, 29B, 27B),and the first and second dielectric gate spacers (52A, 52B) if (a)stress-generating liner(s) is/are not present. Preferably, theplanarization dielectric layer 60 is a dielectric material that may beeasily planarized. For example, the planarization dielectric layer 60can be a doped silicate glass or an undoped silicate glass (siliconoxide).

The planarization dielectric layer 60, the first type stress-generatingliner 58 and/or the second type stress-generating liner 56 (if present),and the dielectric liner 54 (if present) are planarized above thetopmost surfaces of the first and second disposable gate structures(29A, 27A, 29B, 27B), i.e., above the topmost surfaces of the first andsecond disposable gate material portions (27A, 27B). The planarizationcan be performed, for example, by chemical mechanical planarization(CMP). The planar topmost surface of the planarization dielectric layer60 is herein referred to as a planar dielectric surface 63.

In one embodiment, the first conductivity type is p-type and the secondconductivity type is n-type. The first source and drain extensionregions 14A and the first source and drain regions 16A are p-doped, andthe second conductivity type well 12A is n-doped. The combination of thefirst source and drain extension regions 14A, the first source and drainregions 16A, and the second conductivity type well 12A can be employedto subsequently form a p-type field effect transistor. Correspondingly,the first source and drain extension regions 14A and the first sourceand drain regions 16A are n-doped, and the second conductivity type well12A is p-doped. The combination of the first source and drain extensionregions 14A, the first source and drain regions 16A, and the secondconductivity type well 12A can be employed to subsequently form ann-type field effect transistor. The first type stress-generating liner58 can apply a tensile stress to the first channel, and the second typestress-generating liner 56 can apply a compressive stress to the secondchannel.

In another embodiment, the first conductivity type is n-type and thesecond conductivity type is p-type. The first source and drain extensionregions 14A and the first source and drain regions 16A are n-doped, andthe second conductivity type well 12A is p-doped. The combination of thefirst source and drain extension regions 14A, the first source and drainregions 16A, and the second conductivity type well 12A can be employedto subsequently form an n-type field effect transistor. Correspondingly,the first source and drain extension regions 14A and the first sourceand drain regions 16A are p-doped, and the second conductivity type well12A is n-doped. The combination of the first source and drain extensionregions 14A, the first source and drain regions 16A, and the secondconductivity type well 12A can be employed to subsequently form a p-typefield effect transistor. The first type stress-generating liner 58 canapply a compressive stress to the first channel, and the second typestress-generating liner 56 can apply a tensile stress to the secondchannel.

Referring to FIG. 3, the first disposable gate structure (29A, 27A) andthe second disposable gate structure (29B, 27B) are removed by at leastone etch. The at least one etch can be a recess etch, which can be anisotropic etch or anisotropic etch. The etch employed to remove thefirst and second disposable gate material portions (27A, 27B) ispreferably selective to the dielectric materials of the planarizationdielectric layer 60, the first type stress-generating liner 58 and/orthe second type stress-generating liner 56 (if present), and the firstand second dielectric gate spacers (52A, 52B). Optionally, one or bothof the dielectric portions (29A, 29B) can be left by etching selectiveto these layers. The disposable gate structures (29A, 27A, 29B, 27B) arerecessed below the planar dielectric surface 63 and to expose thesemiconductor surfaces above the first channel and the second channel toform gate cavities (25A, 25B) over the semiconductor substrate.

Optionally, a first semiconductor-element-containing dielectric layer31A can be formed on the exposed surface of the second conductivity typewell 12A by conversion of the exposed semiconductor material into adielectric material, and a second semiconductor-element-containingdielectric layer 31B can be formed on the exposed surface of the firstconductivity type well 12B by conversion of the exposed semiconductormaterial into the dielectric material. The formation of thesemiconductor-element-containing dielectric layers (31A, 31B) can beeffected by thermal conversion or plasma treatment. If the semiconductormaterial of the second conductivity type well 12A and the firstconductivity type well 12B includes silicon, thesemiconductor-element-containing dielectric layers (31A, 31B) caninclude silicon oxide or silicon nitride. Thesemiconductor-element-containing dielectric layers (31A, 31B) areinterfacial dielectric layers that contact a semiconductor surfaceunderneath and gate dielectrics to be subsequently deposited thereupon.

Referring to FIG. 4, a contiguous gate dielectric layer 32L and afirst-type work function metal layer 34L including a first metal havinga first work function are sequentially formed for form a stack, frombottom to top, of the contiguous gate dielectric layer 32L and thefirst-type work function metal layer 34L. The contiguous gate dielectriclayer 32L can be a high dielectric constant (high-k) material layerhaving a dielectric constant greater than 8.0. The contiguous gatedielectric layer 32L can include a dielectric metal oxide, which is ahigh-k material containing a metal and oxygen, and is known in the artas high-k gate dielectric materials. Dielectric metal oxides can bedeposited by methods well known in the art including, for example,chemical vapor deposition (CVD), physical vapor deposition (PVD),molecular beam deposition (MBD), pulsed laser deposition (PLD), liquidsource misted chemical deposition (LSMCD), atomic layer deposition(ALD), etc. Exemplary high-k dielectric material include HfO₂, ZrO₂,La₂O₃, Al₂O₃, TiO₂, SrTiO₃, LaAlO₃, Y₂O₃, HfO_(x)N_(y), ZrO_(x)N_(y),La₂O_(x)N_(y), Al₂O_(x)N_(y), TiO_(x)N_(y), SrTiO_(x)N_(y),LaAlO_(x)N_(y), Y₂O_(x)N_(y), a silicate thereof, and an alloy thereof.Each value of x is independently from 0.5 to 3 and each value of y isindependently from 0 to 2. The thickness of the contiguous gatedielectric layer 32L, as measured at horizontal portions, can be from0.9 nm to 6 nm, and preferably from 1.0 nm to 3 nm. The high-k materiallayer 32L may have an effective oxide thickness on the order of or lessthan 1 nm.

The first-type work function metal layer 34L includes a first metal,which has a first work function. The first metal of the first-type workfunction metal layer 34L is selected to optimize the performance of atransistor to be subsequently formed employing the first source anddrain extension regions 14A, the first source and drain regions 16A, andthe second conductivity type well 12A.

In a first embodiment, the first conductivity type is p-type and thesemiconductor material of the second conductivity type well 12A includesn-doped silicon, and the first-type work function metal layer 34Lincludes a silicon valence band edge metals such as Pt, Rh, Ir, Ru, Cu,Os, Be, Co, Pd, Te, Cr, Ni, TiN, and alloys thereof. A silicon valenceband edge metal is a metal having a work function that is closer to thework function corresponding to the valence band edge of silicon, i.e.,5.10 eV, than to the work function corresponding to the conduction bandedge of silicon, i.e., 4.00 eV. Thus, a silicon valence band edge metalhas a work function that is greater than 4.55 eV. For example, thefirst-type work function metal layer 34L can be a layer of TiN.

In a second embodiment, the first conductivity type is n-type and thesemiconductor material of the second conductivity type well 12A includesp-doped silicon, and the first-type work function metal layer 34Lincludes a silicon conduction band edge metals such as Hf, Ti, Zr, Cd,La, Tl, Yb, Al, Ce, Eu, Li, Pb, Tb, Bi, In, Lu, Nb, Sm, V, Zr, Ga, Mg,Gd, Y, and TiAl, and alloys thereof. A silicon conduction band edgemetal is a metal having a work function that is closer to the workfunction corresponding to the conduction band edge of silicon than tothe work function corresponding to the valence band edge of silicon.Thus, a silicon conduction band edge metal has a work function that isless than 4.55 eV. For example, the first-type work function metal layer34L can be a layer of TiAl.

The first-type work function metal layer 34L can be formed, for example,by physical vapor deposition, chemical vapor deposition, or atomic layerdeposition (ALD). The thickness of the first-type work function metallayer 34L is typically set at a value from 1 nm to 30 nm, and moretypically, from 2 nm to 10 nm, although lesser and greater thicknessescan also be employed.

Referring to FIG. 5, a first photoresist 39 is applied and lithographicpatterned so that the first photoresist 39 covers the area over thesecond conductivity type well 12A, while the top surface of thefirst-type work function metal layer 34L is exposed over the firstconductivity type well 12B. The pattern in the first photoresist 39 istransferred into the first-type work function metal layer 34L by anetch. The portion of the first-type work function metal layer 34L withinthe second gate cavity 25B is removed employing the first photoresist 39as an etch mask. The first photoresist 39 is removed, for example, byashing or wet etching. After the patterning of the first-type workfunction metal layer 34L, the first-type work function metal layer 34Lis present in the first gate cavity 25A (See FIG. 4), but is not presentin the second gate cavity 25B.

Referring to FIG. 6, a second-type work function metal layer 36L isdeposited. The second-type work function metal layer 36L includes asecond metal having a second work function, which is different from thefirst work function. The second metal of the second-type work functionmetal layer 36L is selected to optimize the performance of a transistorto be subsequently formed employing the second source and drainextension regions 14B, the second source and drain regions 16B, and thefirst conductivity type well 12B.

In the first embodiment, the second conductivity type is n-type and thesemiconductor material of the first conductivity type well 12B includesp-doped silicon, and the second-type work function metal layer 36Lincludes a silicon conduction band edge metals such as Hf, Ti, Zr, Cd,La, Tl, Yb, Al, Ce, Eu, Li, Pb, Tb, Bi, In, Lu, Nb, Sm, V, Zr, Ga, Mg,Gd, Y, and TiAl, and alloys thereof. For example, the second-type workfunction metal layer 36L can be a layer of TiAl.

In the second embodiment, the second conductivity type is p-type and thesemiconductor material of the first conductivity type well 12B includesn-doped silicon, and the second-type work function metal layer 36Lincludes a silicon valence band edge metals such as Pt, Rh, Ir, Ru, Cu,Os, Be, Co, Pd, Te, Cr, Ni, TiN, and alloys thereof. For example, thesecond-type work function metal layer 36L can be a layer of TiN.

The second-type work function metal layer 36L can be formed, forexample, by physical vapor deposition, chemical vapor deposition, oratomic layer deposition (ALD). The thickness of the second-type workfunction metal layer 34L is typically set at a value from 2 nm to 100nm, and more typically, from 3 nm to 10 nm, although lesser and greaterthicknesses can also be employed.

In the first and second embodiments, one of the first and second workfunctions is closer to the conduction band of the semiconductor materialof the first conductivity type well 12B and the second conductivity typewell 12A than the valence band of that semiconductor material, and theother of the first and second work functions is closer to the valenceband than to the conduction band of that material. Typically, the workfunction that is closer to the conduction band than to the valence bandof the semiconductor material is employed to enhance the performance ofan n-type field effect transistor, and the work function that is closerto the valence band than to the conduction band of the semiconductormaterial is employed to enhance the performance of a p-type field effecttransistor.

Referring to FIG. 7, a second photoresist 139 is applied andlithographic patterned so that the second photoresist 139 covers thearea over the first conductivity type well 12B, while the top surface ofthe second-type work function metal layer 36L is exposed over the secondconductivity type well 12A. The pattern in the second photoresist 139 istransferred into the second-type work function metal layer 36L by anetch. The portion of the second-type work function metal layer 36Lwithin the first gate cavity 25A is removed employing the secondphotoresist 37 as an etch mask during the etch. The second photoresist139 is removed, for example, by ashing or wet etching. After thepatterning of the second-type work function metal layer 36L, thesecond-type work function metal layer 36L is present in the second gatecavity 25B (See FIG. 6), but is not present in the first gate cavity25A.

Referring to FIGS. 8 and 9, an optional at least one work functionadjustment layer 37L can be deposited. Optional at least one workfunction adjustment layer 37 can include one or more optional metalliclayers that can adjust the work functions of the transistors to besubsequently formed on the first conductivity type well 12B and thesecond conductivity type well 12A. As a non-limiting illustrativeexample, the optional at least one work function adjustment layer 37Lcan include a stack, from bottom to top, of a first titanium nitridelayer, a tantalum nitride layer, a second titanium nitride layer, and atitanium aluminum alloy layer. Alternately, the optional at least onework function adjustment layer 37L can include only a subset of thestack of the first titanium nitride layer, the tantalum nitride layer,the second titanium nitride layer, and the titanium aluminum alloylayer, or can even be omitted.

At least one barrier metal layer 38L is deposited on the first-type workfunction metal layer 34L and the second-type work function metal layer36L. In one embodiment of the present disclosure, the at least onebarrier metal layer 38L includes a lower barrier metal layer 138including a metal, a metal oxide monolayer 238 including an oxide of themetal, and an upper barrier metal layer 228 including the metal. Forexample, the lower barrier metal layer 138 can be a titanium nitridelayer, the metal oxide monolayer 238 can be a monolayer of titaniumoxide, and the upper barrier metal layer 338 can be another titaniumnitride layer.

In case the at least one barrier metal layer 38L includes a stack, frombottom to top, of a lower barrier metal layer 138 including titaniumnitride, a metal oxide monolayer 238 including a monolayer of titaniumoxide, and an upper barrier metal layer 338 including titanium nitride,the effect of the presence of the metal oxide monolayer 238 within thestack can have the effect of raising the work function of the at leastone barrier metal layer 38L by up to 70 mV relative to a barrier metallayer including only titanium nitride and having a thickness that isequal to the sum of the thicknesses of the lower barrier metal layer 138including titanium nitride and the upper barrier metal layer 338including titanium nitride layer.

The stack of the lower barrier metal layer 138 including titaniumnitride, the metal oxide monolayer 238 including a monolayer of titaniumoxide, and the upper barrier metal layer 338 including titanium nitridecan be formed by a two-step deposition process with an oxidation processtherebetween. Specifically, a lower titanium nitride layer having athickness from 0.5 nm to 5 nm can be deposited first, followed byexposure of the upper surface of the lower titanium nitride layer to anoxidizing ambient, and then an upper titanium nitride layer having athickness from 1 nm to 10 nm can be deposited.

In one embodiment, the lower titanium nitride layer and the uppertitanium nitride layer can be deposited by physical vapor deposition,and the monolayer of titanium oxide can be formed by exposing the topsurface of the lower titanium nitride layer to an oxidizing ambient. Thedeposition temperature for the lower and upper titanium nitride layerscan be between 20° C. and 450° C., and is typically about 350° C. Theoxidizing ambient for forming the monolayer of titanium oxide can haveoxygen partial pressure of 100 mTorr to 100 Torr at a temperature from20° C. to 600° C., and typically from 350° C. to 450° C., althoughlesser and greater oxygen partial pressures and/ore lesser and greateroxidation temperatures can also be employed. The oxidation time can beadjusted to limit the thickness of the titanium oxide on the surface ofthe lower titanium nitride layer to a monolayer. The formation of thetitanium oxide can be a self-limiting process in which the thickness ofthe titanium oxide formed on the surface of the lower titanium nitridelayer is limited to a single monolayer as long as the predominantmechanism of oxygen incorporation is adsorption to the surface of thelower titanium nitride layer, and a bulk diffusion of oxygen into thelower titanium nitride layer is insignificant.

The effect of the presence of the metal oxide monolayer 238 in the formof the monolayer of titanium oxide in the metal stack is an increase inthe work function of the material stack. This increase in the workfunction can be about 70 mV if the lower titanium nitride layer and theupper titanium nitride layer are deposited by physical vapor deposition.The increase in the work function tends to decrease if the lowertitanium nitride layer and the upper titanium nitride layer aredeposited by atomic layer deposition. In view of the general result ofreduction in the work function due to any oxidation process asillustrated in FIG. 1, this is an unexpected result because thedirection of change in the work function, i.e., the increase in the workfunction, is contrary to the general reduction of work function as aresult of any oxidation processing step introduced to a replacement gateelectrode. Further, the amount of increase in the work function due tothe presence of a monolayer of titanium oxide between two layers oftitanium nitride is significant because such an increase, particularlyin combination of additional methods that increase the work functioneven further, can enhance the performance of a p-type field effecttransistor.

Referring to FIG. 10, a conductive metal layer 40L is deposited on theat lest one barrier metal layer 38L. The conductive metal layer 40Lincludes a conductive material such as aluminum deposited by physicalvapor deposition. High temperature aluminum deposition process at orabout 440° C. is known to provide good reflow characteristics so thatgate cavities can be completely filled upon deposition of the conductivemetal layer 40L.

The conductive metal layer 40L can an aluminum-including layer thatincludes aluminum as a predominant component. For example, thealuminum-including layer can include at least 98% of aluminum in atomicconcentration. The aluminum-including layer can consist essentially ofaluminum so that the electrical conductivity of the conductive metallayer 40L is maximized.

In one embodiment, the aluminum-including layer can be deposited at atemperature that reduces reflow rate of the deposited material. Whilethis method is contrary to the generally known method of enhancing thefill property of the deposited material, the reduction of the depositiontemperature of the aluminum-including material has the unexpected effectof raising the work function of the material stack including thealuminum-including layer. Specifically, when the aluminum-includinglayer including at least 98% of aluminum in atomic concentration isdeposited at a temperature not exceeding 420° C., the work function ofthe material stack including the aluminum-including layer can increasesignificantly. For example, a tested sample including an aluminum layerdeposited at 400° C. by physical vapor deposition in vacuum environmenthad a work function that was 70 mV greater than a control sampleincluding an aluminum layer deposited at 440° C. by physical vapordeposition in vacuum environment and otherwise subjected to the sameprocessing conditions.

While the cause is unclear as to why the work function increases for agate electrode employing an aluminum layer deposited at a lowertemperature relative to the work function of other gate electrodesemploying an aluminum layer deposited at the conventional aluminumdeposition temperature of 440° C., one possible speculation is that thelower deposition temperature affects microcrystalline structure and thegrain size of the deposited aluminum material. Further, the lack ofreflow may have an effect on the increase in the work function of thegate electrode employing the aluminum layer deposited at a lowertemperature. Because some reflow is necessary to fill the gate cavities,however, the deposition temperature for the aluminum layer cannot bedecreased indefinitely. Selection of a deposition temperature not lessthan 380° C. and not greater than 440° C. is a good compromise betweenthe requirement that the work function of the gate stack be increasedsignificantly (e.g., on the order of 70 mV), and the requirement thatthe gate cavities be filled.

Referring to FIG. 11, portions of the conductive material layer 40L, thesecond-type work function metal layer 36L, the first-type work functionmetal layer 34L, and the portion of the contiguous gate dielectric layer32L are removed from above the planar dielectric surface 63 of theplanarization dielectric layer 63 by employing a planarization process.

A first field effect transistor is formed in the region of the secondconductivity type well 12A. The first field effect transistor includesthe second conductivity type well 12A, the first source and drainextension regions 14A, the first source and drain regions 16A, a firstmetal semiconductor alloy portions 64A, the optional firstsemiconductor-element-containing dielectric layer 31A, and a firstreplacement gate stack 230A. The first replacement gate stack 230Aincludes a stack, from bottom to top, of a first gate dielectric 32Awhich is a remaining portion of the contiguous gate dielectric layer32L, a first-type work function metal portion 34 which is a remainingportion of the first-type work function metal layer 34L, a firstoptional work function adjustment portion 37A that is a portion of theoptional at least one work function adjustment layer 37L, a firstbarrier metal portion 38A which is a remaining portion of the at leastone barrier metal layer 38L, and a first conductive material portion 40Awhich is a remaining portion of the conductive material layer 40L. Thefirst second-type work function metal portion 38A includes the secondmetal and contacts the first-type work function metal portion 34 thatincludes the first metal.

A second field effect transistor is formed in the region of the firstconductivity type well 12B. The second field effect transistor includesthe first conductivity type well 12B, the second source and drainextension regions 14B, the second source and drain regions 16B, a secondmetal semiconductor alloy portions 64B, the optional secondsemiconductor-element-containing dielectric layer 31B, and a secondreplacement gate stack 230B. The second replacement gate stack 230Bincludes a stack, from bottom to top, of a second gate dielectric 32Bwhich is a remaining portion of the contiguous gate dielectric layer32L, a second-type work function metal portion 36 which is a remainingportion of the second-type work function metal layer 36L, a secondoptional work function adjustment portion 37B that is a portion of theoptional at least one work function adjustment layer 37L, a secondbarrier metal portion 38B which is a remaining portion of the at leastone barrier metal layer 38L, and a second conductive material portion40B which is a remaining portion of the conductive material layer 40L.

In the first embodiment, the first conductivity type is p-type, thesecond conductivity type is n-type, and the first field effecttransistor is a p-type transistor, and the second field effecttransistor is an n-type field effect transistor. In the secondembodiment, the first conductivity type is n-type, the secondconductivity type is p-type, and the first field effect transistor is ann-type transistor, and the second field effect transistor is a p-typefield effect transistor.

Each of the first and second gate dielectrics (32A, 32B) includes ahorizontal gate dielectric portion and a vertical gate dielectricportion extending upward from peripheral regions of the horizontal gatedielectric portion. The first conductive material portion 40A contactsan upper surface and inner sidewalls of the first second-type workfunction metal portion 38A. The second conductive material portion 40Bcontacts an upper surface and inner sidewalls of the second second-typework function metal portion 36.

Referring to FIG. 12, a contact-level dielectric layer 70 is depositedover the planarization dielectric layer 60. Various contact viastructures can be formed, for example, by formation of contact viacavities by a combination of lithographic patterning and an anisotropicetch followed by deposition of a conductive material and planarizationthat removes an excess portion of the conductive material from above thecontact-level dielectric layer 70. The various contact via structurescan include, for example, first source/drain contact via structures 66A,second source/drain contact via structures 66B, a first gate contact viastructure 68A, and a second gate contact via structure 68B.

While the disclosure has been described in terms of specificembodiments, it is evident in view of the foregoing description thatnumerous alternatives, modifications and variations will be apparent tothose skilled in the art. Accordingly, the disclosure is intended toencompass all such alternatives, modifications and variations which fallwithin the scope and spirit of the disclosure and the following claims.

What is claimed is:
 1. A semiconductor structure comprising a fieldeffect transistor, said field effect transistor including a gate stackcomprising: a gate dielectric located on a semiconductor substrate; alower barrier metal portion comprising a metal and located on said gatedielectric; a metal oxide monolayer portion comprising an oxide of saidmetal and contacting said lower barrier metal portion; and an upperbarrier metal portion contacting said metal oxide monolayer portion. 2.The semiconductor structure of claim 1, wherein said lower barrier metalportion comprises titanium nitride, said metal oxide monolayer portioncomprises a monolayer of titanium oxide, and said upper barrier metalportion comprises another titanium nitride portion.
 3. The semiconductorstructure of claim 1, wherein said gate stack includes a work functionmetal portion contacting said gate dielectric and said lower barriermetal portion.
 4. The semiconductor structure of claim 3, wherein saidgate dielectric includes a dielectric material having a dielectricconstant greater than 8.0, and said work function metal portion includesa metal having a work function that is greater than 4.55 eV.
 5. Thesemiconductor structure of claim 4, wherein said work function metalportion includes a silicon valence band edge metal.
 6. The semiconductorstructure of claim 5, wherein said silicon valence band edge metal isselected from Pt, Rh, Ir, Ru, Cu, Os, Be, Co, Pd, Te, Cr, Ni, TiN andalloys thereof.
 7. The semiconductor structure of claim 3, wherein saidgate dielectric includes a dielectric material having a dielectricconstant greater than 8.0, and said work function metal portion includesa metal having a work function that is less than 4.55 eV.
 8. Thesemiconductor structure of claim 7, wherein said work function metalportion includes a silicon conduction band edge metal.
 9. Thesemiconductor structure of claim 8, wherein said silicon conduction bandedge metal is selected from Hf, Ti, Zr, Cd, La, Tl, Yb, Al, Ce, Eu, Li,Pb, Tb, Bi, In, Lu, Nb, Sm, V, Ga, Mg, Gd, Y, TiAl and alloys thereof.10. The semiconductor structure of claim 1, wherein said lower barriermetal layer has a thickness from 0.5 nm to 5 nm, and said upper barriermetal layer has a thickness from 1 nm to 10 nm.
 11. The semiconductorstructure of claim 4, wherein said gate dielectric is selected fromHfO₂, ZrO₂, La₂O₃, Al₂O₃, TiO₂, SrTiO₃, LaAlO₃, Y₂O₃, HfO_(x)N_(y),ZrO_(x)N_(y), La₂O_(x)N_(y), Al₂O_(x)N_(y), TiO_(x)N_(y),SrTiO_(x)N_(y), LaAlO_(x)N_(y), Y₂O_(x)N_(y), a silicate thereof, and analloy thereof, wherein x is from 0.5 to 3 and y is from 0 to
 2. 12. Thesemiconductor structure of claim 7, wherein said gate dielectric isselected from HfO₂, ZrO₂, La₂O₃, Al₂O₃, TiO₂, SrTiO₃, LaAlO₃, Y₂O₃,HfO_(x)N_(y), ZrO_(x)N_(y), La₂O_(x)N_(y), Al₂O_(x)N_(y), TiO_(x)N_(y),SrTiO_(x)N_(y), LaAlO_(x)N_(y), Y₂O_(x)N_(y), a silicate thereof, and analloy thereof, wherein x is from 0.5 to 3 and y is from 0 to
 2. 13. Thesemiconductor structure of claim 1, further comprises a conductive metallayer located on the upper barrier metal portion of the gate stack. 14.The semiconductor structure of claim 13, wherein said conductive metallayer comprises an aluminum-containing layer.
 15. The semiconductorstructure of claim 14, wherein said aluminum-containing layer comprisesat least 98% aluminum.
 16. The semiconductor structure of claim 14,wherein said aluminum-containing layer consists essentially of aluminum.17. The semiconductor structure of claim 1, further comprising a gatedielectric spacer located on the sidewalls of the gate stack.